Kotlinx deserialization does not include properties from abstract class when using "JsonContentPolymorphicSerializer"

Question

Lets say there is abstract class BaseClass ...

import kotlinx.serialization.SerialName
import kotlinx.serialization.Serializable

@Serializable(with = PolymorphicSerializer::class)
abstract class BaseClass {
    @SerialName("bid")
    var baseId: String? = null
 }

... class ExtendedClass that extends it ...

import kotlinx.serialization.SerialName
import kotlinx.serialization.Serializable

@Serializable
class ExtendedClass
    (@SerialName("eid")
     var newId: String? = null
) : BaseClass()

... and PolymorphicSerializer

import kotlinx.serialization.DeserializationStrategy
import kotlinx.serialization.json.JsonContentPolymorphicSerializer
import kotlinx.serialization.json.JsonElement

object PolymorphicSerializer  : JsonContentPolymorphicSerializer<BaseClass>(BaseClass::class) {
    override fun selectDeserializer(element: JsonElement): DeserializationStrategy<out BaseClass> {
        return ExtendedClass.serializer()
    }
}

PolymorphicSerializer is used because JSON can be deserialized as different classes that extend BaseClass based on some property (for sake of simplicity I just used ExtendedClass.serializer() )

Now, if we try to deserialize generic JSON to ExtendedClass ...

class DeserializationTest {
    @Test
    fun deserialization_isCorrect() {
        val extendedClass = Json {
            ignoreUnknownKeys = true
        }.decodeFromString<BaseClass>("""{"bid":"bid","eid":"eid"}""")

        assertEquals("bid", extendedClass.baseId)
    }
}

... test fails

java.lang.AssertionError: 
Expected :bid
Actual   :null

If I try to serialize ExtendedClass ...

val extendedClass = ExtendedClass()
extendedClass.baseId = "bid"
extendedClass.newId = "eid"

val encodedJson = Json {
    ignoreUnknownKeys = true
}.encodeToString(extendedClass)

I can see encodedJson is {"eid":"eid"} but if I remove JsonContentPolymorphicSerializer this works fine (replace @Serializable(with = PolymorphicSerializer::class) with @Serializable ), encodedJson is {"bid":"bid","eid":"eid"}

So, for some reason JsonContentPolymorphicSerializer does not take into account properties from super class.

Anyone knows why is that so?

Did you find solution? Im thinking of including two properties with the same value, one acting as the selecer - as it's seemingly removed — MikkelT, Nov 29 '22 at 17:18
@MikkelT there is an issue on Github and a workaround is mentioned there: https://github.com/Kotlin/kotlinx.serialization/issues/1433 Unfortunately, still no nice solution for the problem. — Ivan Škugor, Dec 12 '22 at 12:15

score -1 · Answer 1 · answered May 18 '21 at 22:22

-1

You are overriding the generated serializer for BaseClass by providing your custom PolymorphicSerializer:

@Serializable(with = PolymorphicSerializer::class)
abstract class BaseClass { ... }

You have effectively erased the serializer for BaseClass. (In fact, I'm surprised this doesn't throw some type of error/failing deserialization.)

Instead, remove @Serializable, and wherever you need to deserialize JSON which does not contain a class discriminator, pass your custom PolymorphicSerializer as the parameter to decodeFromString.

P.s. I would also recommend to name it differently. PolymorphicSerializer is an actual serializer in kotlinx.serialization which threw me off. ;)

answered May 18 '21 at 22:22

Steven Jeuris

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Hi Steven. Sorry about naming, it was just dummy name, unfortunately a bad one. Regarding your solution, I just tried it, but it does not work, baseId is still null after deserialization. – Ivan Škugor May 24 '21 at 15:29
This doesn't work for me either. Moreover, sometimes you need to declare the custom `JsonContentPolymorphicSerializer` in the base class, for example when you need it to serialize a polymorphic `List< BaseClass >` (from the example above). AFAIK there is no way to implement a custom List serializer that works with polymorphic collections. – fast3r May 17 '22 at 15:19
@fast3r Then you need to create a `ListSerializer` and pass your custom serializer as the parameter. – Steven Jeuris May 17 '22 at 16:50

Kotlinx deserialization does not include properties from abstract class when using "JsonContentPolymorphicSerializer"

1 Answers1